ABSTRACT Age of pubertal onset has declined dramatically in both girls and boys in the US, with substantial racial/ethnic disparities. This has important public health implications as early puberty is linked with a variety of detrimental adolescent behavioral and emotional outcomes as well as adult chronic diseases. In response to these trends, clinicians have adjusted by shifting the guidelines for the cutoff point for early puberty from age <8 to as early as age <6 in girls. To design effective interventions, we must first: (1) establish current normative trajectories for US adolescents by sex, race/ethnicity and BMI to reliably identify children who are going through puberty substantially earlier than their peers of the same racial/ethnic group, and (2) identify sex-specific modifiable risk factors that can serve as targets for intervention strategies in high-risk groups. Healthy People 2020 emphasizes the importance of improving adolescent health because puberty is an important window of susceptibility that impacts adolescent behaviors and adult morbidity. To address the lack of data on pubertal trajectories by racial/ethnic groups, described in PA-18-033, we will take advantage of the robust data for the racially/ethnically diverse population of Kaiser Permanente Northern California (KPNC) to conduct the largest population-based, longitudinal study undertaken to date. Acknowledging the importance of screening for early puberty, KPNC began routine documentation of pediatrician-assessed Tanner stages in the electronic health record (EHR) in 2010, and we have since validated the quality of these data. We will use these pubertal development data, race/ethnicity, and pre-pubertal BMI data to efficiently characterize the sex- and race/ethnicity specific normative distributions of pubertal onset and tempo (rate of pubertal progression from onset to completion of maturation) in 170,000 boys and girls (Aim 1). We will then use data on a sub-cohort of boys and girls who were born at KPNC after the implementation of the EHR system (2006) to create a virtual birth cohort to investigate individual-level early life intervenable metabolic factors such as maternal pre- pregnancy obesity, gestational weight gain, and postnatal growth trajectory (Aim 2) as factors influencing pubertal timing. Lastly, we will link historical geocoded address data to the California Neighborhood Data System, which we developed, to investigate contextual factors (e.g., social and built environment) influencing pubertal timing (Aim 3). Our studies incorporate several innovations and strengths including the use of large and comprehensive EHR and a diverse population of boys and girls to conduct an otherwise costly longitudinal study, examination of several important hallmarks of puberty, and investigation of individual and contextual level factors as risk factors. Risk factors identified from this study and the role of sex and race/ethnicity will guide the design of upstream individual- as well as policy-level interventions. Such interventions targeting high risk groups will advance our long-term goal of preventing health disparities worsened by early puberty.